With respect to a semiconductor device (especially, a silicon device), integration and power reduction of a device have been advanced owing to miniaturization (scaling law; Moore's law), and progress in integration and power reduction at a pace of 4-fold per 3 years has been made. However, the gate length of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) has recently reached 20 nm or less bringing substantial cost increase of a lithography process (device price, and mask set price), and a physical limit of a device dimension (operating limit, or variation limit), and improvement of the device performance by an approach different from the past one depending on the scaling law has come to be sought-after.
A programmable logic device which is reprogrammable called as FPGA (Field Programmable Gate Array), which may be positioned in an intermediate category between a gate array and a standard cell, has been recently developed. With a FPGA, customers themselves can conduct an arbitrary circuit configuration after production of a chip. A FPGA has variable resistance elements, so that customers themselves can establish at their arbitrary electrical connections of wiring. By use of a semiconductor device mounting such a FPGA, the degree of freedom of a circuit can be improved. Examples of a variable resistance element include a ReRAM (Resistance Random Access Memory) using a transition metal oxide, and a solid electrolyte switch or an atomic switch using an ion conductor.
Patent Literature 1 and Non Patent Literature 1 disclose a constitution, an operation, and a crossbar switch in the case of a 2-terminal type switching element (variable resistance element), which regulates the conduction state between two electrodes placed sandwiching an ion conductor (a solid in which an ion can move freely by application of an electric field, or otherwise).
Non Patent Literature 1 discloses a switching element utilizing metal ion movement in an ion conductor and an electrochemical reaction. A switching element disclosed in Non Patent Literature 1 is configured with an ion conductive layer, and a first electrode and a second electrode placed such that the two face each other sandwiching the ion conductive layer. Among these, the first electrode functions as a supplier of a metal ion to the ion conductive layer. The second electrode does not supply a metal ion to the ion conductive layer.
The operation of the switching element will be described briefly. When the second electrode is grounded and a positive voltage is applied to the first electrode, a metal of the first electrode dissolves as a metal ion into the ion conductive layer. Then, the metal ion in the ion conductive layer deposits as a metal in the ion conductive layer and the deposited metal forms a metal-bridge (also called as filament, or conductive path), which connects the first electrode with the second electrode. Due to the electrical connection of the first electrode and the second electrode with the metal-bridge, the switch is put into an ON-state. Meanwhile, in the ON-state, if the first electrode is grounded and a positive voltage is applied to the second electrode, a part of the metal-bridge breaks. As the result, the electrical connection between the first electrode and the second electrode is broken, and the switch is put into an OFF-state. In this regard, at a stage prior to complete breakage of an electrical connection, electrical properties start changing such that the resistance between the first electrode and the second electrode increases, the interelectrode capacitance changes, or the like, and finally the electrical connection breaks. Further, in order to change the OFF-state to an ON-state, the second electrode is grounded again, and a positive voltage is applied to the first electrode.
The switching element is characterized in that the size is smaller than a semiconductor switch (MOSFET, etc.), and the ON-resistance (resistance in an ON-state) is low. Therefore, the switching element is considered to have promise for application to a programmable logic device. Since the conduction state (ON or OFF) of the switching element can be maintained as original without applying a voltage, an application as a nonvolatile memory element is also conceivable. For example, a memory cell containing a selective element such as a transistor and a switching element is used as a basic unit, and a plurality of the memory cells are aligned in each of a longitudinal direction and a cross direction. By the alignment, an optional memory cell can be selected among a plurality of the memory cells by means of a word line and a bit line. Thus, a nonvolatile memory can be realized, in which the stored information of either of “1” or “0” can be read from the state of ON or OFF of the switching element by sensing the conduction state of the switching element of the selected memory cell.